VIEW ARTICLE

Ecology and Epidemiology

Alteration of Physiological Processes in Wheat Flag Leaves Caused by Stem Rust and Leaf Rust. M. T. McGrath, Former graduate research assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802, Present address: Long Island Horticultural Research Laboratory, Cornell University, 39 Sound Avenue, Riverhead, NY 11901; S. P. Pennypacker, Associate professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 80:677-686. Accepted for publication 18 July 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-677.

The severity of rust on flag leaves of winter wheat (Triticum aestivum ‘Tyler’) infected by Puccinia recondita f. sp. tritici, the causal agent of leaf rust, and/or P. graminis f. sp. tritici, the causal agent of stem rust, was associated with decreased apparent photosynthetic rates per unit of leaf area (PRA), photosynthetic rates per unit of chlorophyll (PRC), chlorophyll content (CC), ratio of chlorophyll a/chlorophyll b (A/B), and transpiration rate (TR). Severity was also associated with increased internal CO₂ concentration (C_i) and stomatal resistance (R_s). These response variables were measured on attached leaves of field-grown plants during the grain-filling period. Variability due to time of observation was removed by expressing each observation as relative to the value predicted for a rust-free leaf at the same time. The relative response variables were linearly related to the base 10 logarithm of rust severity plus one. Leaves with 9% stem rust in 1986 were predicted by regression analysis to exhibit a 66% reduction in PRA, a 42% reduction in PRC, a 54% reduction in CC, an 8% reduction in A/B, a 20% reduction in TR, a 10% increase in C_i, and an 88% increase in R_s as compared with rust-free leaves. Leaves with 9% stem and/or leaf rust in 1987 were predicted to exhibit a 60% reduction in PRA, a 35% reduction in PRC, a 54% reduction in CC, a 12% reduction in A/B, an 18% reduction in TR, a 23% increase in C_i, and a 68% increase in R_s. These two diseases affected their host similarly. Loss of photosynthetic tissue on an area basis could not account for the reduced photosynthetic rate since the percent reduction greatly exceeded the percent leaf area covered by uredinia. This reduced rate was partially due to decreased chlorophyll content.